Что такое Transaction ID Wraparound в PostgreSQL?

Transaction ID Wraparound in PostgreSQL is a situation related to the limit on the number of unique transaction identifiers (Transaction IDs, or XID) that PostgreSQL can use.

Each transaction receives a unique 32-bit XID, which increments. Due to the limited range (about 4 billion), XIDs can "wrap around," meaning the counter will return to zero and start reusing old values.

If the database does not perform regular cleanup (vacuum), old XIDs can be mistakenly perceived as new, leading to data visibility issues and potential integrity loss.

To prevent wraparound, PostgreSQL automatically runs autovacuum, which updates transaction visibility information and prevents XID reuse as long as it is safe.

If wraparound occurs, it can lead to "freezing" tables and the need for emergency intervention.

Thus, Transaction ID Wraparound is a limitation of PostgreSQL's architecture, requiring regular database maintenance to prevent errors.